Most consumer electronic products require a power supply unit, such as an adapter and a charger, as a power source or charging the batteries thereof. However, in no-load condition or the batteries already fully charged, the internal circuitry of the power supply unit remains in operation and therefore still consumes electric power. The accumulated power consumption can be considerable if the power supply unit is left turned on for a long time.
In a conventional power supply unit, as shown in FIG. 1, a switching power converter 10 converts an input voltage Vin into an output voltage Vout under control of a controller 12 to provide for load. In order to reduce power consumption, the power supply unit will enter a burst mode at light loading or no loading, in which the power switch Qsw will skip several switching cycles as long as the output voltage Vout is sufficiently high, and the operation current of the controller 12 will be lowered, thereby reducing the total power consumption of the controller 12 and the switching power converter 10. In addition, the photocoupler 14 at the secondary side uses one having a higher current transfer ratio (CTR) to generate a same feedback current Ic under a lower forward bias current If, and the voltage stabilizer circuit 16 uses a shunt regulator 17 having a smaller operation current, such as TLV431, to reduce the operation current of the feedback network in a constant output voltage Vout mode, and thereby reduce the power consumption of the entire feedback network. However, a photocoupler 14 having a high current transfer ratio needs a shunt regulator 17 having a smaller operation current, and thus the circuit requires higher component costs and the photocoupler 14 may have stability issue due to a lower forward bias current If.